1. Field of the Invention
The present invention relates to the field of power plant engineering. It pertains to a method of controlling and regulating a power plant, in which power plant thermal power is generated in a combustion chamber from a gaseous fuel and the thermal power is at least partially converted into electrical power in conversion equipment, and in which power plant the fuel for the combustion chamber is produced in a fuel producer from a feed product with the application of thermal and/or electrical power, which is removed at the outlet from the conversion equipment, and said fuel is passed on to the combustion chamber.
The present invention also relates to a power plant for implementing the method.
2. Discussion of Background
In view of the fact that environmental protection legislation is becoming increasingly more strict, it is becoming increasingly more difficult to dispose of waste products from refinery processes. For this reason, processes which convert such waste products into fuels which can subsequently be used in a power plant to generate electrical and/or thermal power are becoming profitable.
A typical plant for the conversion of waste materials into electrical energy comprises a gasification plant which converts the tar accumulating as refinery waste into so-called "Syngas", a combustible mixture of various gases. The Syngas is then burned in a gas turbine, which at least partially converts the thermal energy of the hot gases produced into electrical energy (Integrated Fuel and Power Generation IFPG). In this case, the gas turbine is usually part of a combined process (with a gas and steam turbine), in order to increase the overall efficiency of the conversion of Syngas into electrical power. Some of the power generated is fed back (in the form of steam and/or electrical power) into the gasification plant, in order to effect the production of the Syngas. There is, therefore, close coupling between the Syngas producer (gasification plant) and the Syngas consumer (gas turbine or power plant block). In particular, stable, steady-state operation of these closely interwoven parts of the plant can be achieved only if the production and consumption of the Syngas are in harmony with each other. To this end, it is necessary for a control and regulation method to be developed which takes account of the special features of an IFPG plant and differs considerably from the operation of an autonomous power plant (Standalone Power Plant SAPP), in which the fuel is obtained from a store in the form of a gas or oil tank.
The typical construction of a power plant 10 in the form of a conventional SAPP plant is reproduced in schematic form in FIG. 1. A fuel supply device 12 (e.g. a pump) removes the fuel from a fuel store 11 and compresses or decompresses (expands) said fuel to a specific predefined pressure p.sub.f1. The fuel supply to the combustion chamber 16 is controlled by a control valve 14. A fuel distribution system 15 distributes the fuel mass flow fed in to one or more burners arranged inside the combustion chamber 16. The thermal power from the combustion chamber 16 is then converted, in subsequent conversion equipment 17 (gas turbine, waste-heat recovery boiler, steam turbine) into electrical power and/or steam, for example in a combined cycle. The basic strategy for controlling and regulating such a plant has two objectives: (a) the production of any desired, time-variable output power profiles within the limits of the operating range of the plant, and (b) ensuring the necessary fuel supply without significant delays.
Objective (a) is typically achieved by a power control loop, which brings the tracking error EQU dP(t)=P.sub.c (t)-P.sub.m (t)
to zero during steady-state operation. P.sub.c (t) designates the possibly time-variable set point for the power, and P.sub.m (t) is, according to FIG. 1, the measured output power. An overview of the SAPP plant with the principal control loops is illustrated in FIG. 1.
A significant component of the control system is the power controller 20. The power controller 20 may contain further control loops internally (for example temperature controllers, pressure regulators etc.), which are necessary to keep the internal states of the power plant within the prescribed operating limits. The power controller may optionally regulate the thermal power (steam) output by the conversion equipment 17 at the outlet 18, or the electrical power output at the output 19. On the input side, the power controller 20 has applied to it the difference between P.sub.c and P.sub.m, which is formed in a subtracter 21. On the output side, said power controller 20 outputs a signal which corresponds to the required fuel mass flow m.sub.fc. The output signal from the power controller 20 is converted, in a subsequent fuel/valve position converter 22, into a valve position signal h.sub.c for the control valve 14 (or a corresponding variable of another fuel control member, such as the desired speed of a variable-speed fuel pump). The fuel mass flow through the control valve 14 depends on the pressures p.sub.f1 and p.sub.f2 upstream and downstream of the valve, the measured values of these pressures also being input into the fuel/valve position converter 22 for the purpose of calculating h.sub.c. For an inlet pressure p.sub.f1 within the limiting curves a and b illustrated in FIG. 2, and within the operating limits of the control valve 14, it is thus possible for any desired fuel mass flow to be set, virtually without delay, by suitable selection of the valve position signal h.sub.c. The significant limitation in the dynamics of the mass flow is imposed by the dynamics of the control valve 14. The valve must therefore be designed such that it satisfies all the requirements for achieving the abovementioned objective (a). In order to achieve the abovementioned objective (b) (a precondition for the objective (a)), the fuel supply device 12 must ensure that the input pressure pfl is kept within the limiting curves a and b illustrated in FIG. 2. To this end, according to FIG. 1 a pressure control loop for p.sub.f1 is provided, and is composed of the fuel supply device 12 and a pressure regulator 13. It should be noted at this point that the fuel supply device 12 is typically a fuel delivery pump, a gas compressor or a pressure reducing valve.
By comparison with the structure of an SAPP plant shown in FIG. 1, an IFPG plant, in which the fuel is produced exclusively within the plant itself, and on which the method of the present invention is based, has the basic structure reproduced in FIG. 3. Such a power plant 30 is characterized by a fuel producer 31, a reducing device 32 (in some cases this may also be a controllable compression device) and a power plant block as has already been described in connection with FIG. 1 and which comprises a control valve 33, a fuel distribution system 34, a combustion chamber 35 and conversion equipment 36 having an outlet 37 for thermal power (steam) and an outlet 38 for electrical power. The reducing device 32 may be a controllable pressure-reducing valve or any other controllable pressure-reducing device. From the outlets 37 and 38 of the conversion equipment 36 (for example gas turbine, waste-heat recovery boiler and steam turbine), a steam feedback line 391 and a power feedback line 392 for electrical power provide the fuel producer 31 with the energy necessary for fuel production. Via the supply 393, an appropriate feed product (for example tar) is fed into the fuel producer 31, to be converted into fuel.
The control and regulation of the IFPG plant according to FIG. 3 differs in principle from the control and regulation of the SAPP plant from FIG. 1, since, in the IFPG plant, the fuel producer 31 must be controlled in such a way that, within the operating limits of the power plant, it is adapted to the fuel consumption of the conversion equipment 36. The setting of the objective for the control may in this case be described such that for the power there is a control loop in which the production and the consumption of fuel are adapted to each other within predefined limits. The limits may be expressed in the form of operating pressures.
Those operating limits for the plant which are significant and important for the control are the limiting pressures for the pressure Pfl at the inlet to the control valve 33 and for the pressure p.sub.f3 at the outlet of the fuel producer 31. The limiting curves a and b reproduced in FIG. 2 likewise apply to both pressures, said curves representing intrinsic properties of the fuel producer 31 and of the conversion equipment 36, and it being possible for said curves to depend on various plant parameters, such as the electrical output power generated. The operation of the plant 30 is only permissible when p.sub.f1 and p.sub.f3 are within the corresponding limits. If these limits are exceeded, emergency measures must be taken for the fuel producer 31 and/or the conversion equipment 36. There are further boundary conditions for the operation of the fuel producer 31 and conversion equipment 36, these conditions being taken into account by internal controllers, although it is not necessary to discuss these further in the present connection.
The control and regulation system which is suitable for a plant according to FIG. 3 must satisfy the following objectives:
(1) It must stabilize the pressure p.sub.f3 at the outlet of the fuel producer 31 within predefined limits (operating condition for the fuel producer). PA1 (2) It must stabilize the inlet pressure p.sub.f1 for the fuel distribution system 34 within predefined limits (operating condition for the fuel distribution system). PA1 (3) It must bring the fuel production and the fuel consumption into harmony with each other, in order to obtain steady-state operating points within the permissible operating range for any desired and demanded output power from the conversion equipment 36. PA1 (4) It must manage the transients of the entire system, including start-up and shutdown. PA1 (1) for m.sub.PG &gt;m.sub.FG (fuel consumption higher than fuel production), the pressure values for p.sub.f1 and p.sub.f3 will decrease if there is no corrective intervention and the fuel production rate is constant (since the supply of fuel available in the fuel producer and fuel distribution system decreases), until said pressure values depart in a downward transgression from the operating range defined by the limiting curves b in FIG. 2. PA1 (2) for m.sub.PG &lt;m.sub.FG (fuel consumption lower than fuel production), the pressure values for p.sub.f1 and p.sub.f3 will rise if there is no corrective intervention and the fuel production rate is constant (since fuel accumulates in the storage volume of the fuel producer and of the fuel distribution system), until said pressure values exceed the limiting curve a in FIG. 2. PA1 (1) There is only a weak dynamic correlation between the output power and the supply of the feed product to the fuel producer. In particular, during the production of fuel the quality (the calorific value) of the fuel produced may be subjected to severe fluctuations. Over and above this, there are a large number of further parameters which influence the quality of the fuel. The supply of the feed product therefore represents a very imprecise measure of the power produced. PA1 (2) The dynamics of fuel production (of the fuel producer 31) are generally very slow in comparison with the power dynamics of the power plant block and the conversion equipment 36. The power controller 42 must therefore be adapted to the dynamics of the fuel producer 31, which leads to a narrow bandwidth for the stabilization of the output power. However, this is inadequate for applications such as frequency response, island operation or emergency deloading of the power plant along predefined power gradients. PA1 (3) Combined power plants usually have standard block control systems which are built up on the basic loop from FIG. 1. The power variables involved (measured power P.sub.m, desired power P.sub.c) are used to distribute the power generation to different power generation units (for example a number of gas turbines or steam turbines) within the power plant, each of the units being controlled by its own power controller. The lack of a power control loop for the power plant in the solution according to FIG. 4 consequently requires the concept of the standard block control system to be rearranged. This becomes clear from the fact that the control valve 33, which (as control valve 14) is used as an actuator for the output power of the power plant in the solution according to FIG. 1, is now assigned to a pressure control loop in the solution according to FIG. 4. PA1 (4) Nor does the control structure according to FIG. 4 take any account of the significant relationships between input variables and output variables which are necessary for the stabilization of the individual parts of the plant: the pressure p.sub.f3 is an important parameter for the dynamics of the fuel producer 31. Conversely, the supply of the feed product to the fuel producer 31 is the principal actuator for the stabilization of p.sub.f3. On the other hand, the output power is the principal parameter for the stabilization of the power plant, the fuel supply to the power plant block being the principal controlled variable. It is obvious that the controller structure shown in FIG. 4 does not take these relationships into account, which may lead to problems in the form of dynamic instabilities.
The fundamental difficulty which is common to the abovementioned objectives is that of maintaining the equilibrium between the fuel mass flows of the fuel consumption m.sub.PG and the fuel production m.sub.FG, namely EQU m.sub.PG =m.sub.FG.
Each deviation from this equilibrium results in a transient in the system and leads to an infringement of the operating conditions if appropriate countermeasures are not taken. In the event of a deviation from equilibrium, two cases may be distinguished:
The type of control and regulation system used previously in an IFPG plant according to FIG. 3 is reproduced in the simplified schematic diagram in FIG. 4. The significant feature of the control loop for such a power plant 40 is a power controller 42, whose output acts on the supply 393 of the feed product for the fuel producer 31. In this regard, the control system differs from the control system of the SAPP plant from FIG. 1, where the output power from the power plant is controlled via the control valve 14. In order to stabilize the plant within the operating limits, pressure regulators 43 and 44 for the pressures p.sub.f1 and p.sub.f3 are also needed.
However, this known control system has various disadvantages: